Processes are known for fabrication of bipolar transistors having an extrinsic base region self-aligned to the sacrificial emitter structure. One example of such a process is described by M. Racanelli et al. in an article entitled “Ultra High Speed SiGe NPN for Advanced BiCMOS Technology”, in the IEDM-2001 proceedings. Link base and extrinsic base implants are blocked from penetration in the intrinsic device area by the sacrificial emitter feature and the sacrificial emitter feature with a side wall spacer, respectively. Heavy p-type implants required to convert epitaxially deposited base material into a low resistivity extrinsic base region result in the generation of interstitial defects in the single crystal portion of the extrinsic base. The presence of interstitial defects promotes the transient enhanced diffusion of boron from the extrinsic base region into the intrinsic base during subsequent thermal processing. Interstitial defects also promote transient diffusion of the boron incorporated in the epitaxial base layer which leads to the effective base widening and the device speed reduction.
Another problem associated with the fabrication of higher speed bipolar transistors is the need to form thinner base films, which directly translates into a high base resistance. FIG. 1 illustrates a cross-sectional view of a typical npn bipolar transistor. The extrinsic link base region (Rbshl) comprises a thin Si, SiGe or SiGe:C epitaxial layer as used in the intrinsic base. Although heavily doped, this region will have higher resistance for thinner base films (faster transistors).

The graph shown above illustrates the calculated dependence of base resistance Rb on the link base sheet resistance (Rbshl) for a given device geometry. A factor of three reduction in the link base resistance (Rbshl) will result in a significant (approximately 80 ohm) reduction in base resistance (Rb).
In view of the above, it is apparent that there is a need to provide a high speed bipolar transistor and a method of fabricating a high speed bipolar transistor which reduces or avoids the above mentioned problems.